The effect of lactic acid as a determinant of bacterial and enzymic changes is very important. Early in the ripening process, lactic acid suppresses the growth of undesirable micro-organisms. It also furnishes the acid medium necessary for the best action of both the coagulating and peptic enzymes. The importance of the lactic acid bacteria in cheese ripening has been summed up by Hastings[117] as follows: "The functions of this group of bacteria in Cheddar cheese are through their by-product lactic acid as follows: (_a_) To favor the curdling of milk by rennet. (_b_) The bacteria of the milk are held in great part in the curd. Through the acid they influence the shrinkage of the curd and expulsion of the whey, (_c_) The acid so changes the nature of the curd as to cause 'matting,' or 'cheddaring' of the curd, (_d_) The acid activates the pepsin of the rennet extract, (_e_) The acid prevents the growth of putrefactive bacteria in the cheese. (_f_) It has been shown that _Bacterium lactis acidi_ is able to form acid in the absence of the living cell. (_g_) The development of _Bacterium lactis acidi_ is followed by the growth of another group of acid-forming bacteria, the _Bacillus Bulgaricus_ group. They reach numbers comparable with those of the first group, reaching their maximum number within the first month of ripening. Since they develop after the fermentation of the milk-sugar, they must have some other source of carbon and of energy than milk-sugar." It is also probable that other groups constantly present contribute to the changes. From the preceding discussion it is evident that each of the ripening agents has its important part to play in the ripening process and a normal ripening of the cheese is a composite result of these various agencies. +239. Conditions affecting the rate of cheese ripening.+--The rate at which these agents cause ripening of the cheese depends on several factors.[118] Most of these factors are within the control of man. They are as follows: the length of time; temperature of the curing-room; moisture-content of the cheese; size of the cheese; the quantity of salt used; the amount of rennet; the influence of acid. +240. The length of time.+--The water-soluble nitrogen compounds increase as the cheese ages, other conditions being uniform. The rate of increase is not uniform; it is much more rapid in the early than in the succeeding stages of ripening.